Temperature dependence of the resistance for CsV\(_3\) Sb\(_5\)  thin flakes with various thickness, from bulk single crystal to 4.8 nm, are shown in Fig. 2. The data are normalized by the values at 290 and 140 K for (a) and (b), respectively. The curves are vertically shifted for clarity. The arrows mark the CDW transition temperature \({\it T}_{\rm CDW}\) . For the bulk single crystal, \({\it T}_{\rm CDW}\)  \(\approx \)  94 K is consistent with previous report [1]}. With decreasing of the thickness, \({\it T}_{\rm CDW}\)  decreases from 94 K in bulk to a minimum of 82 K at thickness of 60 nm, then increases to 120 K as the thickness is further reduced to 4.8 nm (about five monolayers). Such a non-monotonic evolution of \({\it T}_{\rm CDW}\)  with sample thickness in CsV\(_3\) Sb\(_5\)  is quite unusual. To our knowledge, only VSe\(_2\)  was reported to manifest this kind of non-monotonic thickness dependent of \({\it T}_{\rm CDW}\)  [2]}. It was attributed to the crossover in the Fermi surface topology from 3D to 2D around 20 nm [2]}. For bulk CsV\(_3\) Sb\(_5\) , 2 \(\times \)  2 \(\times \)  2 superstructure [4]} and 2 \(\times \)  2 \(\times \)  4 [5]} superstructure were reported, by means of X-ray diffraction, suggesting 3D CDW. Therefore, the non-monotonic evolution of \({\it T}_{\rm CDW}\)  with sample thickness in CsV\(_3\) Sb\(_5\)  may be also due to a crossover from 3D to 2D around 60 nm. Note that 60 nm is actually quite thick, far from the 2D limit.
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